Australia were just a fraction ahead of Sweden when Emma McKeon hit the water halfway through the 4x100 metres freestyle relay final.

The race was effectively over moments later, the Wollongong star producing a stunning 51.35 split.

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It was the fifth fastest relay leg of all time and left jaws dropped across the arena.

From there, Cate Campbell's anchor leg was simply a procession, the four-time Olympian reaching the wall three seconds ahead of her nearest rivals and in front of the world record line.

Canada finished second, with the USA in third.

It marked Australia's first gold medal of the Games and the third-straight Olympics the squad has won this event.

Bronte Campbell led the team off, before handing over to Meg Harris.

The pair set the race up for McKeon and she didn't disappoint, blowing her rivals away to put her country in a commanding position.

For McKeon, the swim was simply what her teammates needed with gold on the line.

"Being part of this relay always lifts you," McKeon told Channel 7.

"Going out with the girls you always know you have that extra bit of adrenaline. I actually felt more relaxed going in with the rest of the team.

"Seeing the girls put up a really good heat swim and all six of us are part of this team, it's very exciting."

While McKeon was unwilling to talk the swim up, the performance was a warning to her rivals.

She's in Tokyo to collect a record medal haul.

Sportsbet has McKeon listed as the $1.72 favourite for the 100m freestyle final and after Sunday's swim it's not hard to see why.

The athlete has a more immediate focus, however, with her first shot at solo glory coming on Monday morning.

McKeon will contest the 100m butterfly final, having finished second in Sunday's semi-final to safely progress to the medal race.

Her time of 56.33 seconds was slower than the heat, but it came on the back of an Australian record on Saturday night.

Among her rivals are China's Yufei Zhang, while defending champion Sarah Sjoestroem and American teenager Torri Huske will also be in the mix for the medals.

A medal in the 100m butterfly final will mark the second of a potential seven throughout the competition.

Such an achievement would make McKeon the most successful Australian at a single Games and our most decorated athlete of all time.

Australia's relay victory came on the back of a successful morning in the pool.

Brendon Smith claimed the country's first medal with bronze in the men's 400m individual medley.

Jack McLoughlin then went one better to secure silver in the 400m freestyle.

The Queenslander finished just 0.16 seconds behind Tunisia's Ahmed Hafnaoui, the 18-year-old claiming a stunning upset victory from lane eight.

"I just knew they were coming at me, so I was just hoping I could put my hand on the wall," McLoughlin said.

"I am super stoked for second, a little annoyed I didn't get the win.

"It was my first international medal. Last year I was pretty close to quitting so to be here now is pretty surreal and I'm very happy."